Catalytic process for producing diarylmethanes



United States Patent r CATALYTIC PROCESS FOR PRODUCING DIARYLMETHANES Lloyd C. Fetter-1y, El Cerrito, Calif., assignor'to Shell Development Company, New York, N.Y., a corpora-- tion of Delaware No Drawing. Application December 27, 1955 Serial No. 555,313

7 Claims. (Cl. 260-668) This invention relates to a process for the production of diarylmethanes, and more specifically to a process for producing diarylmethanes by condensation of alkyl aromatic hydrocarbons with formaldehyde in the presence of a porous, solid, acidic catalyst.

It is an object of the present invention to provide an improved process for the production of condensation products of aromatic hydrocarbons and formaldehyde containing essentially only diarylmethanes and no more than a low concentration of triaryldimethanes and higher condensation products. It is a further object to prepare diarylmethanes by the reaction of formaldehyde with alkyl-substituted hydrocarbons in the presence of a novel catalyst therefor. Other objects of the present invention will appear from the following description thereof.

The present invention provides an improved method for converting alkyl-substituted aromatic hydrocarbons having at least one unsubstituted nuclear carbon atom into diarylmethanes by condensation with formaldehyde in a process suitable for commercial use. Briefly, the present invention provides a process for the production of diarylmethanes by contacting formaldehyde with a suitable aromatic hydrocarbon feed in the presence of a catalyst consisting of silica gel of high surface area, having deposited thereon from 0.001 to 3.0 millimoles, per gram of silica, of a strong mineral acid which is substantially nonvolatile at a temperature of 200 C.

Suitable hydrocarbon feedstocks for the present invention are alkyl-substituted monocyclic aromatic hydrocarbons containing at least one unsubstituted nuclear carbon atom. The reactivity of aromatic hydrocarbons with formaldehyde under the conditions of the present invention increases with the increasing number of alkyl substituents in the ring. Benzene is not readily converted to diphenylmethane according to the present invention. Monocyclic alkyl aromatic hydrocarbons containing at least one unsubstituted nuclear carbon atom and having no more than six carbon atoms in any one alkyl group and no more than fifteen carbon atoms per molecule are preferred feedstocks. Especially preferred feedstocks are the monoto pentamethylbenzenes. Aromatics having alkyl substituents other than methyl groups can be employed including, for example, ethylbenzene, ethyltoluenes, ethylxylenes, diethylbenzenes, cumene, iso propyltoluenes, isopropylxylenes and the like. The aromatic hydrocarbon feedstocks may be employed as a relatively pure fraction of a single compound or of a single molecular weight range, *but mixtures of aromatic hydrocarbons may also be employed.

Formaldehyde can be employed'in the present invention in aqueous or anhydrous form. It may be charged 2,879,312 Patented Mar. 2.4, 19:59

ice

, matic solvents, as charge stocks to a hydrocracking step to produce methylatedaromatics, or as intermediates in the production of insecticides or of wetting agents.

The diarylmethane compounds resulting from the use of toluene, mixed Xylenes, pseudocumene, mesitylene and mixed tetramethylbenzenes as feedstocks are suitably hydrocracked in accordance with the method describedin US. patent application Serial No. 399,570 of L. C. Fetterly, filed December 21, 1953, now Patent No. 2,819,322, to produce respectively in substantial yield paraxylene, pseudocumene, durene, isodurene and pentamethyl-v benzene.

It is known to produce diarylmethanes by reaction of aromatic hydrocarbons with catalysts consisting essentially of concentrated'sulfuric acid. These prior art catalysts have the undesirable tendency of causing loss of aromatic hydrocarbon feed to by-products such as sulfonates. Furthermore, these catalysts are not suitable for use with an aqueous solution of formaldehyde such as formalin, which is the most readily and economically available commercial form of formaldehyde.

It has now been found that by use of catalystsconsist-j ing, essentially of silica gel having deposited thereon no more than a monomolecular layer of strong mineral acid;

centration of sulfuric acid on the silica gel is between 0.04 and 0.25 millimole/g. SiO the range between 0.05 and 0.1 millimole/g. Si0 being especially suitable. The correlation between the amount of sulfuric acid-deposited on silica gel and the activity of the catalyst is illustrated in Table I, in which the formaldehyde conversion obtain.- able with a freshcatalyst is shown for silica gel containing various amounts of sulfuric acid deposited thereon.

TABLE I Amount of H1804, Millimolespergram SiOz..- 0 0.01 0.05 0.25 0.5 1.0

Hydrocarbon Feed Formaldehyde Conversion, Percent xylenes (mixed) 5-- s7 94 93 9o pseudoeumene 30 92 GIN These results were obtained with the'respec'tive identified hydrocarbon feedstocks under conditions which were. substantially identical for any one feedstock;the reactions were carried out in the manner described in Example I, The formaldehyde was employed inthe form of' 37% aqueous formalin. It is apparent that formaldehyde conversion is highest withcatalysts"having between 0.05 and 1 rnillirnole H SO /g. SiO .It was also observed, how- TABLE II Catalyst life, Grams alkylata per grameat yst 2 s 10 12 1 1 1e 20 25 Formaldehyde Conversion, Percent The sulfuric acid-on-silica gel catalyst is simply prepared by spraying dry silica gel with the desired amount of an aqueous solution of sulfuric acid. It is preferred to use a'relatively dilute acid solution in order to permit uniform application of the acid to the gel. Use of a solution of 1% to 10% by weight H 80 is therefore preferred, a strength of about 5% by weight being very suitable. If the amount of water added to the gel by spraying with aqueous acid does not exceed about 30% by weight, the gel may be directly used in the alkylation step. If more water is employed, the gel should be dried before use, e.g.,by heating at 150 C. for one hour or more. Although the resulting catalyst contains sulfuric acid and may also contain water, it has the feel and appearance of dry silica gel.

Other mineral acids, e. g., phosphoric acid, phosphotungstic and silicotungstic acid, supported on porous silica gel in the. same concentration range as sulfuric acid (as millimoles/g. SiO are successfully employed in catalyzing the condensation of alkyl aromatic hydrocarbons with formaldehyde to form diarylmethanes. Such catalysts are prepared from aqueous solutions of the acid in the same manner as described for sulfuric acid catalysts.

The solid acidic catalysts found effective in the process of the present invention have in common a relatively high surface area and porosity, a pronounced amount of acidity (as measured by titration with an organic base), and a substantial absence of acid sites-having an acid strength as great as that of concentrated liquid sulfuric acid (as measured by means of appropriate indicators). The surface area and porosity of porous solids usually are related, the solids of highest surface area having the smallest pore sizes. Thus, catalyst having a surface area between 500 and 800 or more square meters per gram and average pore diameters between 20 and 50 Angstrom units have been found particularly useful in the present process, while those of lower surface area, between 100 and 500 m. /g., having an average pore diameter between- 50 and 100 or more A., were generally less active, but still useful wtih the more reactive feedstocks.

In order to be effective, the catalyst surface must be acidic, but not excessively so. The preferred catalysts consist of siilca gel of the preferred surface area and porosity, having deposited thereon no more than one half theamountv equivalent to. a monomolecular layer of a. polybasic mineral acid which is substantially nonvolatile at the reaction conditions, e.g., H 50 H PO or phos' photungst'icacid. It is estimated that 3 millimoles of H SQ per gram SiO on. a silica. gel of 800 mP/g. surface area. corresponds. toa monomolecular layer. When an amount of. normally liquid mineral acid in excess of monomolecuuar layer is employed the excess amount of acid acts like the'concentrated mineral acid itself, e.g., like a catalyst of liquidI-IQSO as indicated by the fact that such a catalyst contains a substantial amount of acidity having a pK lower (more negative) than -8. (A pK of 8 corresponds to a liquid sulfuric acid of about 90% concentration; pK of 9 corresponds to 97% H A solid catalyst of such strong acidity is unsuitable for use in the present process. In the preferred catalysts, at least about of the acid sites of the catalyst should have a pK no lower than 8.2.

The extreme limits of concentration of the mineral acid on silica gel of high surface area are from 0.001 to 3 millimoles per gram of silica; the preferred range is from 0.01 to 1.5 millimoles per gram of silica, and the range between 0.04 and 0.25 millimole per gram of silica is especially preferred. One millimole of H 50 per gram of SD; is equal to about 10% by weight H 50 based on SiO Also, one millimole of a mineral acid per gram SiO is equal to 0.01 millimole per square meter for a gel of 100 mF/g. surface area, and to 0.00125 millimole per square meter for a gel of 800 mfi/gQsurface area.

In the process of reacting an alkyl aromatic hydrocarbon with formaldehyde, the solid acidic catalyst may be come contaminated by carbonaceous deposits. The catalysts employed according to the present invention are readily regenerated by a conventional oxidative regeneration, e.g., by burning the catalyst with a gas containing a controlled amount of free oxygen such as air. Part or all of the mineral acid may be lost from the silica gel during regeneration. If that is the case the acid is readily replaced, in the manner described above, before the catalyst is returned to the reaction zone.

In a particularly suitable method of carrying out the present reaction the catalyst is employed in the form of finely divided particles which are slurried in a liquid mass' of aromatic hydrocarbon feed which, during the course of the reaction, also will include the condensation prodnets of the reaction. In a preferred method of operation an agitated slurry comprising the catalyst particles suspended in the liquid is maintained in a heated reaction zone at a temperature sufficiently high to permit prompt removal of water, added to and formed in the reaction zone, in the form of a vapor stream comprising the water and some of the charge hydrocarbon. Aqueous or anhydrous formaldehyde is gradually added to the reaction zone; any water which is added with the formaldehyde, together with the Water formed in the reaction, is immediately removed from the reaction zone by continuously withdrawing vapors of water and aromatic. The vapors withdrawn from the reaction zone are condensed; the aromatic hydrocarbon is suitably returned to the reaction zone. If unreacted formaldehyde is removed in the vapor stream it will be contained in the water layer of the condensate; such recovered formaldehyde may also be returned to the reaction zone. The reaction can be carried out in a batchwise manner, e.g., by placing a desired amount of the aromatic hydrocarbon in the reaction zone together with the required amount of catalyst, agitating and heating and gradually adding sufficient formaldehyde to produce the desired amount of the diarylmethane. The reaction can also be carried out continuously by maintaining a body of liquid comprising catalyst slurried in aromatic hydrocarbon charge and product in the reaction zone, adding fresh aromatic hydrocarbon charge and formaldehyde and withdrawing a bleed stream of the liquid for removal of product therefrom and return of the remainder to the reaction zone.

When operating in the above-described manner, reaction temperatures between 100 C. and 200 C. are pre ferred although temperatures up to 250 C. may be employed. Temperatures between C. and C. are most suitable. Atmospheric pressure is preferably employed although it may be desirable to employ somewhat higher pressures to permit operating at higher temperatures, particularly with a relatively low-boiling hydrocarbon such as toluene. Thus, pressures from one to ten atmospheres are suitably employed while the pressure 15 of from one to three atmospheres is generally preferred. In the above described method of operation it is preferable to add the formaldehyde gradually and to maintain a high ratio of feed aromatic hydrocarbon to unreacted formaldehyde monomer in the reaction slurry, e.g., from 30 to 2,000 moles of aromatic per mole of formaldehyde. In a continuous reaction system the composition of the steady state reaction mixture is controlled ficient to cause vapor of water and aromatic hydrocarbon to be continually withdrawn from the reaction zone through a reflux condenser which returned the hydrocarbon to the reaction zone and permitted the water to be removed. The operating conditions and results of these runs are given in Table III. Component analyses of mixed hydrocarbon feeds employed in some of these runs are shown 'in Table IV.

TABLE III Feed Aromatic Condensation Product 9 Reaction Time Volume Conver- Ulti- Temperfor Ratio, slon of Run Catalyst Bolling mate ature, 5For- Aromatormal- Diaryl-Triaryl- .Heavier Pt. or Molar C. malin ics: dehyde, methdi. (as Tetra- Range, Ratio Addl- Catalyst Percent ane, metharyltri-' C. tion. Y Percent ane, methane),

min. Percent Percent Toluene 110. 6 7. 1/1 99-103 30 5/1 63 Paraxylene- 4. /1 134-136 87 /1 63 Xylenesm- 6. l/1 132-137 84 /1 94 Xylenes 6. 1/1 133-138 55 5/1 90 Trimethylbenzenesm 162-176. 5 5. 5/1 150-160 34 5/1 92 Trimethylbenzenes 162176.5 5.4/1 160-164 35 10/1 76 Mesitylene (99 164.6 2.7/1 1 150-158 105 6/1 78 Durene 95% 193-5 4. 7/1 155-160 59 ca. 5/1 96 Silica Gel+10 Prehnitene 90%) 204 1. 9/1 148-160 103 5/1 89 Silica Gel+10% wt. H28 0| Tetramethylben- 193-204 3. 3/1 155-158 162 5. 5/1 93 zenes *Details in Table YA. 'lotal heavier than diarylmethane, calculated as triaryldimethaue. I to maintain in the liquid no more than 60% by weight of TABLE IV condensation product and preferably less than 50% and 39 Aromatic eed com 0st 1 desirably as low as to Similarly in a batch I f p reaction the addition of formaldehyde is discontinued R N U 3 4 5 6 when the concentration of the condensation product in un] the liquid has reached 60% by weight, or earller. Ammatiw Although the above-described manner of carrying out Ethyl'benzene 4 the reaction is particularly suitable, the solid acidic catao r ne (1. '-d1methy1benzene) 5 M '1 lyst may aliso bei empley dhl a relictlon dgt zl thle/ iii-gi i'igtifiiiieigftije i 3 formaldehy e an aromatic y rocar on are a e simu 6111mm 1 we r e y enzene P d 1,3,4-t 1m 1 taneously to the reaction zone in a batch reaction or 1t Mi sit iififi ii,ihmmth iiiiiiliiiiff? ig may be employed in carrying out the react1on between gg ig -e y y b 3 formaldehyde and arolmatic hydrocadrbonhin vapor phase. a 1 2 5t g l g The amount of cata yst maintaine in t e reaction zone 80 r8119 6 m y enzene i liquid phase operation is in the range between one D-urene(12'4'5'tetmmethy1benzane) 24 and thirty weight percent or more, and preferably between I ten and twenty weight Precent of the hydrocarbon pres The data in Table III illustrate the effectiveness of the ent in the reaction zone.

In numerous runs carried out in accordance with the present invention it has been found that the amount of aromatic hydrocarbon feed reacted was substantially entirely converted to condensation product with formaldehyde, i.e., to the extent of 98% or better. With minor exceptions, the condensation product obtained consisted of at least 85% and generally between 90 and 100% of the diarylmethanes the remainder being mainly triaryldimethanes and sometimes small amounts of the tetraaryltrirnethanes or higher compounds. Substantially no resins were produced in the reaction according to the present method.

The present invention will be further described by means of the following illustrative examples, which are not to be considered as limitative of the invention but merely are presented to illustrate some aspects thereof.

EXAMPLE I A number of runs were carried out employing catalysts comprising silica gel of high surface area and from 0.1 to 1.0 millimole of sulfuric acid per gram SiO (from 1 to 10% by weight) and employing different methylbenzene hydrocarbons from toluene through tetramethylbenzenes. The runs were carried out by placing into a reaction vessel a desired amount of the aromatic hydrocarbon and of the finely divided catalyst, heating the mass in the reaction vessel and stirring it to produce a slurry, and then gradually adding aqueous formalin (37% aqueous formaldehyde) while maintaining an elevated temperature sufcatalyst of the present invention in catalyzing the condensation of aromatic hydrocarbon and formaldehyde for a variety of hydrocarbons. Other conditions being equal, the conversion of formaldehyde is a measure of the reactivity of the hydrocarbon and of the catalyst since the unconverted formaldehyde is mainly that which is withdrawn from the reaction zone together with the vapor stream. When the catalyst is effective and the aromatic hydrocarbon is reactive, the formaldehyde reacts quickly so that little or none is lost in the vapor stream, whereas with less active catalyst or aromatic hydrocarbon more formaldehyde is withdrawn and the conversion is low. Since runs 5 and 8, for example, show that 1 millimole of sulfuric acid per gram S103 on silica gel is a highly effective catalyst, the low formaldehyde conversions in runs 1 and 2 are attributed to the relatively lower reactivity of the toluene and paraxylene feed. The relatively low conversion in run 7 with mesitylene is explained by the higher ultimate molar ratio of aromatic to aldehyde, which permitted a greater amount of aldehyde to be lost in the vapor stream.

EXAMPLE II consisting of 0.05 millimole H 50 per gram on a silica gel having a surface area of 800 square meter/gram and average pore diameter of '24 A. and, in the other case, with a similar catalyst having a surface area-of 300 to 400 square meter/ gram and average pore diameter of 80 to 100 A. The aldehyde addition time for the former, highly porous catalyst was 22 minutes and for the latter, moderately porous catalyst 62 minutes, thus favoring increased aldehyde conversion in the latter case. The formaldehyde conversion observed with the former catalyst was 89% and with the latter catalyst, 47%.

EXAMPLE III Although sulfuric acid is the preferred mineral acid for use on a porous solid support in the present invention, other mineral acids supported on a porous carrier are suitably employed. For example, under otherwise identical conditions similar to Example I, toluene and 37% aqueous formalin were reacted in the presence of a catalyst consisting in one case of 10 weight percent sulfuric acid on highly porous silica gel and in the other case of 10 weight percent phosphotungstic acid on identical silica gel. Aldehyde conversions obtained were 55% and 58%, respectively, thus demonstrating substantially identical activity in these catalysts.

In another experiment, mixed xylenes and 37% aqueous formalin were reacted in the presence of a catalyst, in one case, of 10 weight percent sulfuric acid on highly porous silica gel, and, in the other case, of 10 weight percent phosphoric acid on highly porous silica gel. It was found that with the sulfuric acid catalyst, formaldehyde conversion obtained was 84%, and with the phosphoric acid catalyst, 63%, thus demonstrating substantial activity in the silica gel supported phosphoric acid catalyst.

In a further similar experiment a silica gel of high surface area and small pore size, containing no acid deposited thereon, was contacted with trimethylbenzene and an aqueous 37% formalin at conditions similar to run No. of Table III and only 6% of formaldehyde conversion was observed, based on theweight'percent of alkylated aromatics recovered. The weight balance of formaldehyde for this experiment indicated that some aldehyde was reacted, apparently to form polymers of formaldehyde itself rather than diarylmethaues or higher condensation product.

EXAMPLE I" In a run carriedout in the same 1115555315 run No. 1 of Example I, a catalyst was employed which consisted of 1 millimole of sulfuric acid per gram of support on a base which was a fresh commercial cracking catalyst of the aluminum silicate type, containing 12% Al O and 88% Si0 and having a surface area of about 600'sq. m./gram. In this run, essentially no aldehyde conversion was obtained.

I claim as my invention:

1. In the production of diarylmethanes to the substantial exclusion of resins by reaction of an alkyl-substituted aromatic hydrocarbon having at least one unsubstituted nuclear carbon atom and formaldehyde, the improvement which comprises carrying out the reaction in the presence of a catalyst of from 0.05 to 1.5 millimoles, per gram of silica, of a strong mineral acid supported on a porous silica gel.

2. In the production of diarylmethanes to the substantial exclusion of resins by reaction of an alkyl-substituted aromatic hydrocarbon having at least one unsubstituted nuclear carbon atom and formaldehyde, the improvement which comprises carrying out the reaction in the presence of a catalyst of from 0.05 to 1.5 millimoles of sulfuric acid per gram of silica deposited on silica gel of at least sq. m./ gram surface area.

3. A process according to claim 2 in which said silica gel has a surface area of at least 500 sq. m./gram and said amount of sulfuric acid is between 0.05 and 0.10 millimoles per gram of SiO 4. A process according to claim 1 in which said aromatic hydrocarbon is .toluene.

5. A process-according to claim- 1 in which said aromatic hydrocarbonis a xylene.

6'. A process according to claim 1 in which said aromatic hydrocarbon is a tetramethylbenzene.

7. A process according to claim 1 in which said aromatic hydrocarbon is a trimethylbenzene.

References Cited in the file of this patent UNITED STATES PATENTS 2,186,022 Holm et a1. Ian. 9, 1940 2,398,825 Funsten Apr. 23, 1946 2,430,803 Ciapetta Nov. 11, 1947 2,660,572 Feasley Nov. 24, 1953 FOREIGN PATENTS I 446,450 Great Britain Apr. 30, 1936 OTHER REFERENCES Welch et aL, Jour. Amer. Chem. Soc., vol. 73, pp. 4391-3 (1951). 

1. IN THE PRODUCTION OF DIARYLMETTHANES TO THE SUBSTANTIAL EXCLUSION OF RESINS BY REACTION OF AN ALKYL-SUBSTITUTED AROMATIC HYDROCARBON HAVING AT LEAST ONE UNSUBSTITUTED NUCLEAR CARBON ATOM AND FORMALDEHYDE, THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE REACTION IN THE PRESENCE OF A CATALYST OF FROM 0.05 TO 1.5 MILLIMOLES, PER GRAM OD SILICA, OF A STRONG MINERAL ACID SUPPORTED ON A POROUS SILICA GEL. 